A polycrystalline silicon wafer applied in a conventional polycrystalline silicon solar cell is typically formed by cutting a polycrystalline silicon ingot. The manufacture of the polycrystalline silicon ingot is carried out by heating a silicon material in a crucible to obtain molten silicon, which is then cooled down and allowed to condense so as to form the polycrystalline silicon ingot.
A polycrystalline silicon wafer of a solar cell having low resistance and high photoelectric conversion efficiency may be manufactured by adding boron as a dopant during a polycrystalline silicon ingot growing process. However, if a high concentration of oxygen impurity is present during the polycrystalline silicon ingot growing process, the solar cell manufactured thereby may form boron-oxygen defect after light illumination so that the photoelectric conversion efficiency may be reduced.
The crucible for forming the polycrystalline silicon ingot is made of quartz and may release oxygen impurity from the wall thereof at high temperature. The oxygen impurity released from the wall of the crucible may blend into the molten silicon, especially a portion of the molten silicon proximate to the wall of the crucible. In order to solve the aforesaid problem, it is disclosed in, for example, JP 2001-198648 to form a silicon nitride (Si3N4) layer on an inner surface of the crucible so as to easily release the formed polycrystalline silicon ingot from the crucible and to inhibit blending of the oxygen impurity into the molten silicon. However, it is demonstrated that there is still the oxygen impurity diffusing through the silicon nitride layer and blending into the molten silicon. Therefore, the concentration of the oxygen impurity in the molten silicon may not be effectively reduced merely by the silicon nitride layer.
There is thus a need in the art to provide a method to reduce the concentration of the oxygen impurity blended in the polycrystalline silicon ingot to a satisfactory extent so as to permit the polycrystalline silicon ingot to be suitable for manufacturing a polycrystalline silicon wafer of a solar cell having low resistance and high photoelectric conversion efficiency.